PyNodes, the bare basics
PyNodes do not make it possible to extend Blender Internal, Cycles or the compositor with new nodes.

They are designed to make user interfaces for new node systems. For example developers of external render engines can use them to make their own shader node systems, or an addon could make a new geometry node system.

It would of course be very useful if they did, but that would be a much bigger project, as it requires each node system to expose an API with access to its inner workings. For Cycles the closest you can get is Open Shading Language nodes.